Portal: Update the instructions to split content/infrastructure

[psaiz]

The code for the portal has been moved to a different repo (https://github.com/cernopendata/cernopendata-portal, currently private).

The goals of this ticket are:

1. Remove from opendata.cern.ch all the files that provided by that cernopendata-portal
2. Make cernopendata-portal public
3. Update the instructions on this repo, specifying which contributions should go to which repo and how to test them

[psaiz]

I've been looking at some of the last commits, thinking about how they would work in a model with different repos for code/data. Let me put here a couple of example:

• Things that modify only cernopendata/modules/fixtures/data (like this). These ones are easy. They only update the data repo
• Things that modify data and content (like this). The changes of the schema/templates should go to the infrastructure, the one for the data go the content. Two pull requests
• Things like this (record and images). At the moment, these would also require two pull requests, since the images are part of the infrastructure. It would be nice if the images could be provided also as part of the data, and this one required changes only in one repo

[psaiz]

Obsoleted by #3678